A so-called multi-tube reactor is in essence a shell-and-tube exchanger containing up to several thousands or even tens of thousands of substantially vertical reactor tubes inside its shell, each reactor tube containing a fixed bed of catalyst particles and being cooled externally by a fluid circulating between the tubes in the shell. Multi-tube reactors are used for highly exothermic reactions, such as the epoxidation of ethylene. While the cross-sections of the reactor tubes are relatively small (such as 20-50 mm), their length is great (such as 1.5 to 20 m). Inside the reactor shell, the reactor tubes are held together by an upper and a lower tube sheet. Above the tube sheet, the reactor shell forms an upper dome in which maintenance work can be performed, such as the loading and re-loading of the reactor tubes with catalyst. In some reactors the upper dome is removable.
The loading or re-loading of the multitude of narrow and elongated reactor tubes with catalyst, the particles of which are generally not very much smaller than the inner diameter of the tubes, is difficult and time-consuming. An even distribution of the catalyst particles inside each tube and between all tubes is very important but difficult to achieve. During loading it is essential that the number of particles entering the reactor tube at the same time, multiplied by their greatest dimension, be small enough in relation to the internal diameter of the reactor tube so as to avoid the condition known as “bridging.” “Bridging” occurs when several particles enter and fall down the tube simultaneously, wedge together part way down the tube and leave a void space below them—resulting in unevenly and incompletely loaded tubes. When loading the elongated reactor tubes described above, it is best to ensure that the particles enter these tubes one by one. A further requirement, in particular in the ethylene epoxidation reaction which involves gaseous reactants and which is very exothermic, is that a small upper portion of each reactor tube is kept free of catalyst.
In the past it was conventional to place, in effect, a funnel at the upper end of each reactor tube and pour the particles into the individual tubes. Such a procedure is unacceptable today because of the large number of tubes which have to be filled.
U.S. Pat. No. 3,223,490, issued 14 Dec. 1965, discloses a reactor tube loader which comprises (a) a perforated plate which rests on the reactor tubes, the perforations corresponding to the pattern and spacing of the reactor tubes; and (b) fill tubes, one for each reactor tube, which nest in the perforated plate and extend into the corresponding reactor tubes. In operation, catalyst is dumped onto the perforated plate and the plate is shaken by a vibrating mechanism, causing the catalyst particles to pass one by one through the fill tubes and into the reactor tubes. The same publication adds that the fill tubes may be made of such length that when they are loaded to their top with catalyst and then removed from the reactor tubes, their content fills the reactor tubes up to a predetermined point below the top thereof.
GB-B-2186209, issued 1 Feb. 1989, also discloses a reactor tube filling device consisting of a plate resting on the reactor tubes and fill tubes nesting in the plate and extending into the corresponding reactor tubes. The differences with the first document are that the fill tubes are firmly connected to the plate and that a vibrating mechanism is not mentioned. The function of the device according to this document is to ensure that all reactor tubes are filled to a fixed level below their top. The phenomenon of bridging is not mentioned.
The above catalyst loading devices have serious disadvantages. In particular, they are inflexible in that a plate and its associated filling tubes can only be used in a multi-tube reactor of the same size and shape, having the same number, pattern, spacing and diameter of reactor tubes. They are also big, heavy and cumbersome to transport and to introduce into the upper reactor dome.
It is an object of the present invention to provide a much simpler and more flexible loading system for multi-tube reactors. This object is achieved by using a multitude of discrete polygonal plates as defined below, to close-pack the upper tube sheet in a two-dimensional array, i.e. to entirely cover any shape and size of upper tube sheet, in the same way as tiles are used to cover a floor. Together, the polygonal plates form an exceedingly simple and flexible multi-tube loading device.